The invention relates to a valve arrangement in a hydraulic circuit, its use and an arrangement for controlling a hydraulic vehicle drive. In hydraulic control systems there is a need for a valve arrangement which acts as a throttling valve when the pressure of the hydraulic fluid flowing through exceeds a specific threshold value. One important application of this is arrangements for controlling hydraulic vehicle drives. The latter generally have a closed hydraulic circuit which has an variable displacement pump driven by a diesel engine, a hydraulic motor driving the drive wheels of the vehicle and two lines connecting the variable displacement pump and the hydraulic motor for the hydraulic fluid, it being possible for each of the lines, depending on the direction of travel of the vehicle to be the flow or the return line for the hydraulic motor (DE 199 30 997 A1). In hydraulic circuits of this type, the hydraulic motor can act as a pump when the vehicle changes from the normal driven state to trailing or overrun operation. The vehicle then attempts to accelerate the variable displacement pump and therefore also the diesel engine, which can lead to damage to the diesel engine.
For this reason, it has already been proposed, according to DE 199 30 997 A1, to insert into the return line of the hydraulic circuit a pressure regulating valve which acts as a throttle and which acts in the event of increased pressure of the hydraulic fluid flowing from the hydraulic motor to the variable displacement pump. In the pressure regulating valve acting as a throttle, the energy of the hydraulic fluid flowing from the hydraulic motor to the variable displacement pump is partly converted into heat, which means that further undesired overrunning of the diesel engine is prevented. If, during normal driving operation, the usual lower pressures are present in the return line, the throttling action remains nonfunctional. However, if the direction of travel of the vehicle is reversed, the return line then has to serve as a flow line, which is under increased pressure and in which a throttling action would be undesirable. For this reason, in the known vehicle drive according to DE 199 30 997 A1, a line with a nonreturn valve is provided in parallel with the valve arrangement, so that the pressure regulating valve remains inactive when the flow direction is reversed. If it is intended to secure the vehicle against overrunning of the diesel engine in both directions of travel, then the arrangement described of the pressure regulating valve with the parallel-connected nonreturn valve is required in a dual arrangement. This parallel connection of the two functions by means of valves which are designed separately and independently of each other needs a great deal of installation space or leads to a restriction of the flow cross sections, associated with corresponding hydraulic-mechanical power losses.
The invention is therefore based on the object of providing a valve arrangement which acts as a throttle in only one flow direction, with which installation space is saved and at the same time the effort for mounting is reduced, in addition the greatest possible flow cross sections becoming possible in a small available installation space.
According to the invention, this object is achieved in accordance with claim 1 by a valve arrangement in a hydraulic circuit, having a longitudinal duct which opens into a transverse duct, having a tubular throttle slide which can be displaced in some regions in the longitudinal duct and which is acted on in the longitudinal duct with relatively different penetration depths in the direction of one of two different end positions by the hydraulic fluid flowing through the above arrangement, depending on its falling above or below a specific threshold pressure, having lateral passage openings in the wall of the throttle slide, which are arranged in a plurality of groups located with an axial spacing from one another and, in the end position of lower penetration depth of the throttle slide, are connected to the transverse duct while, in the end position of greater penetration depth, a first group of passage openings, facing the penetrating end of the throttle slide, is covered by the wall of the longitudinal duct, and having a tubular valve piston of stepped design which can be displaced in the throttle slide and, depending on its position in the throttle slide, blocks or enables the flow of hydraulic fluid through the interior of the valve piston in the region between the first group of passage openings and the remaining passage openings, a throttling action of the valve arrangement occurring as a result of intermediate positions of the valve slide between its two end positions and in the blocking position of the valve piston when hydraulic fluid under a pressure higher than the threshold pressure flows through the valve arrangement in the direction from the transverse duct to the longitudinal duct.
In claim 1, the threshold pressure means a specific pressure value of the hydraulic fluid flowing through the valve arrangement, which effects the change in the position of the throttle slide if, starting from a higher or lower pressure value, it is reached during the operation of the valve arrangement according to the invention.
The valve arrangement according to the invention is distinguished by the fact that hydraulic fluid under increased pressure can flow through it in both directions but a throttling action occurring only in one flow direction. The throttling action does not occur if the flow against the throttle slide is from one end, to be specific even if it assumes the end position of greater penetration against in accordance with the increased pressure of the hydraulic fluid. In this case, although the first group of its passage openings are covered, that is to say closed, the valve piston changes automatically from the blocking position into its enabling position and permits the hydraulic fluid to flow into the transverse duct via the remaining passage openings without hindrance. In this way, a line with a nonreturn valve led in parallel can be saved.
With the valve arrangement according to the invention, the same action is thus achieved as with the nonreturn valve located in a bypass line in an arrangement according to the prior art. However, the outlay on construction is lower; space is saved and larger flow cross sections can be implemented.
The fact that a pilot control valve is provided in the valve arrangement according to the invention presents no obstacle to this, since the said pilot control valve is expedient in any case for registering a specific threshold pressure of the hydraulic fluid.
Advantageous refinements and structural details of the valve arrangement according to the invention are specified in the independent claims.
The invention also covers the use of a valve arrangement according to claims 1 to 16 in a closed circuit of a hydraulic drive.
In this case, this can be, for example, the drive of a machine tool which is equipped with a hydraulic transmission. A particularly preferred field of application of the valve arrangement according to the invention is, however, vehicle drives. Therefore, the invention also extends to the arrangement for controlling a hydraulic vehicle drive.
The arrangement for controlling a hydraulic vehicle drive, according to the invention, is specified in claim 18 and comprises a closed hydraulic circuit which has an variable displacement pump driven by a diesel engine, a hydraulic motor driving the drive wheels of the vehicle and lines connecting the variable displacement pump and the hydraulic motor for the hydraulic fluid, it being possible for each of the lines to be the flow or return line of the hydraulic motor, depending on the direction of travel of the vehicle, a valve arrangement according to one of claims 1 to 16 being provided in at least one of the lines.
If the vehicle is to be secured against impermissibly high rotational speed of the diesel engine in overrun operation in both directions of travel, a valve arrangement according to the invention can be incorporated in each of the lines.